The present invention relates to an altitude compensation device for internal combustion engines, and more particularly to an altitude compensation device for compensating for an air-fuel ratio of a mixture to be introduced into the engine in accordance with a change of an altitude of the engine.
A known system for controlling a choke valve in the internal combustion engine incorporates a diaphragm serving as an actuator for actuating the choke valve. The diaphragm defines one major wall of a diaphragm chamber which is communicated with an intake pipe of the engine through the intake passage. As the vacuum in the intake passage is increased, the diaphragm is deflected to increase the opening degree of the choke valve. In this choke valve control system, however, the operation characteristic of the diaphragm is not changed even when the altitude of the engine is changed. This causes a problem as follows. Namely, assuming here that the altitude of the engine is increased, the level of the intake vacuum is lowered naturally to reduce the force of the actuator so that the choke valve is held at an opening degree which is smaller than the required one, so that the air-fuel mixture is made excessively rich undesirably resulting in an increase of noxious exhaust emissions and deterioration of the engine performance.